1. Field of the Invention
This invention relates to a system for improving the Safety Integrity Level (SIL) rating of process heaters (e.g., industrial heating systems) using partial stroke testing of an emergency shut-off valve coupled to a supplementary fuel control valve.
2. Description of the Related Art
In the oil, gas, petroleum and power industries, natural gas or other combustible gas is often used to provide the required heat or combustion motive power for the desired operation, more generally referred to herein as a “process.” Various conditions may occur that necessitate immediate shut down or tripping of the operations, process or plant. In those industries, a majority of the final control elements of a shut-off system are implemented with fast acting shut-off valves. In such industries, a majority of the shut-off valves remain open while the process is in a safe and controlled state. Such valves are closed only upon a plant trip, i.e., actuation of the shutdown system of the plant, arising from an out-of-control process or during a normal maintenance outage.
In practice, the testing of emergency shut-off valves is normally done during shut down of the process or plant operation. However, there is a tendency for such valves to stick or freeze due to corrosion or other reasons, which may lead to an unsafe condition where the valve cannot be closed during an emergency shutdown. This problem is exacerbated by economic conditions in the operation of the factory or plant that have lead to a reduction in the frequency of valve shut-offs for maintenance or testing purposes. For example, in some operations a process or operation may run continuously for one or more years without shutting down the process for maintenance.
State of the art emergency shut-off systems, which control the shut-off valves, have a number of features to detect plant or process failures and typically include redundancies for added reliability. However, such systems may not provide for the testing of a shut-off valve itself other than stroking the valve. The problem is that full stroking or completely closing the valve causes an undesirable disruption in the process.
A number of patents have been issued in the past that relate to the operation or stroking of industrial valves, and the increase of reliability in such systems. An example of such is found in U.S. Pat. No. 6,155,282, issued to Zachary et al., which discloses an arrangement for testing solenoids individually without process interruption. The arrangement operates in a two out of three manner to provide relatively high safety, low spurious tripping and a relatively low installation cost, while also enabling on-line testing of each solenoid individually without process interruption. Solenoid arrangements are preferably manifolded to facilitate maintenance on any detected failure, and to simplify installation and replacement.
A more recent U.S. Pat. No. 6,920,409, issued to Essam, discloses an apparatus for testing the operation of an emergency valve. As disclosed, the apparatus is for testing an emergency valve in which a valve member is moveable by a fluid operated actuator between a normally open or closed position and an emergency position, closed or open respectively. The apparatus comprises partial stroking means for the valve including means for initiating emergency movement of the valve from its normal position towards its emergency position, and means for returning it from a predetermined position intermediate the normal and emergency positions at its normal operating speed. The apparatus includes means for detecting the loss of the controlling electrical signal to the valve and a means for detecting the correct operation of the valve and its associated operating components by measuring the pressure of fluid being released from or being applied to the actuator.
In addition, U.S. Pat. No. 7,010,450, issued to Law et al., discloses a combination of field device operations with overrides and bypasses within a process control and safety system. The process control or safety instrumented system uses function block logic to coordinate the logic within the process control or safety instrumented system with operational states of field devices, even when these operational states are initiated externally to the process control or safety system. Logic within input or voter function blocks associated with field devices may monitor and determine when the associated field devices are being put into testing or calibration modes and may automatically initiate appropriate bypass or override functionality in response to such detected field device configuration states. Likewise, the function block logic may automatically remove the bypass or override functionality when the field devices are placed back into their normal operational configuration states. This automatic initiation of bypasses and overrides helps to prevent a safety system within a process plant from initiating a shut-down procedure as a result of a device test initiated manually by, for example, a handheld device attached to a field device. Likewise, the automatic removal of bypasses and overrides helps to prevent a safety system within a process plant from failing to operate properly because a user forgot to manually reset a bypass or override that was set up to allow a device test.
Recognizing that the emergency shut-off valves can be stroked or operated through the partial length or movement of their full travel as a safeguard against frozen or stuck valves has lead to a need for a simple, secure and reliable system for testing such valves without adversely affecting production. This approach also improves the safety of the operation.
The partial stroke testing system in accordance with my earlier invention, described in my U.S. Pat. No. 6,435,022, which is hereby incorporated by reference in its entirety, provides a low cost, simple and reliable test for emergency shut-off valves in the oil, gas, petrochemical and power industries. Such tests do not adversely interrupt the plant or factory operation or process and will minimize or almost eliminate the risk of a “frozen” emergency shutoff valve in the event of an out-of-control process or operation. Such a system is cost effective and has been designed to utilize a shut-off valve with a fast acting piston actuator. The system is also applicable to slow acting valves.
In processes in the oil, gas, petroleum, and power industries, fuel to power the heaters, boilers, or the like, or perhaps to provide fuel for a prime mover, is fed to the associated burners or other components through a fuel control valve and a fuel shut-off valve. The fuel control valve regulates pressure/flow to the burners, whereas the shut-off valve is normally in a fully open state. The shut-off valve closes to cut off fuel supply in the event of an emergency.